This invention relates generally to a white light illumination system, and specifically to a ceramic phosphor blend for converting UV radiation emitted by a light emitting diode (“LED”) to white light.
White light emitting LEDs are used as a backlight in liquid crystal displays and as a replacement for small conventional lamps and fluorescent lamps. As discussed in chapter 10.4 of “The Blue Laser Diode” by S. Nakamura et al., pages 216-221 (Springer 1997), incorporated herein by reference, white light LEDs are fabricated by forming a ceramic phosphor layer on the output surface of a blue light emitting semiconductor LED. Conventionally, the blue LED is an InGaN single quantum well LED and the phosphor is a cerium doped yttrium aluminum garnet (“YAG:Ce”), Y3Al5O12:Ce3+. The blue light emitted by the LED excites the phosphor, causing it to emit yellow light. The blue light emitted by the LED is transmitted through the phosphor and is mixed with the yellow light emitted by the phosphor. The viewer perceives the mixture of blue and yellow light as white light.
However the blue LED—YAG:Ce phosphor white light illumination system suffers from the following disadvantages. The prior art blue LED—YAG:Ce phosphor system produces white light with a high color temperature ranging from 6000K to 8000K, which is comparable to sunlight, and a typical color rendering index (CRI) of about 70 to 75. In other words, the chromaticity or color coordinates of this system are located adjacent to the Black Body Locus (“BBL”) between the color temperatures of 6000K and 8000K on the CIE chromaticity diagram illustrated in FIG. 1. The color temperature of this system can be reduced by increasing the phosphor thickness. However, the increased phosphor thickness decreases the system efficiency.
While the blue LED—YAG:Ce phosphor illumination system with a relatively high color temperature and a relatively low CRI is acceptable to customers in the far east lighting markets, the customers in the North American markets generally prefer an illumination system with a lower color temperature, while the customers European markets generally prefer an illumination system with a high CRI. For example, North American customers generally prefer systems with color temperatures between 3000K and 4100K, while European customers generally prefer systems with a CRI above 90.
The chromaticity coordinates and the CIE chromaticity diagram illustrated in FIG. 1 are explained in detail in several text books, such as pages 98-107 of K. H. Butler, “Fluorescent Lamp Phosphors” (The Pennsylvania State University Press 1980) and pages 109-110 of G. Blasse et al., “Luminescent Materials” (Springer-Verlag 1994), both incorporated herein by reference. The chromaticity coordinates (i.e., color points) that lie along the BBL obey Planck's equation: E(λ)=Aλ−5/(e(B/T)−1), where E is the emission intensity, λ is the emission wavelength, T the color temperature of the black body and A and B are constants. Color coordinates that lie on or near the BBL yield pleasing white light to a human observer. CRI is a relative measurement of how the color rendition of an illumination system compares to that of a black body radiator. The CRI equals 100 if the color coordinates of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the black body radiator.
Another disadvantage of the blue LED—YAG:Ce phosphor system is that the LED color output (e.g., spectral power distribution and peak emission wavelength) varies with the band gap width of the LED active layer and with the power applied to the LED. During production, a certain percentage of LEDs are manufactured with active layers whose actual band gap width is larger or smaller than the desired width. Thus, the color output of such LEDs deviates from the desired parameters. Furthermore, even if the band gap of a particular LED has the desired width, during LED operation the power applied to the LED frequently deviates from the desired value. This also causes the LED color output to deviate from the desired parameters. Since the light emitted by the system contains a blue component from the LED, if the color output of the LED deviates from the desired parameters, then the light output by the system deviates form the desired parameters as well. A significant deviation from the desired parameters may cause the color output of the system to appear non-white (i.e., bluish or yellowish).
Furthermore, the color output of the blue LED—YAG:Ce phosphor system varies greatly due to frequent, unavoidable, routine deviations from desired parameters (i.e., manufacturing systematic variations) during the production of the LED lamp because the color output of this system is very sensitive to the thickness of the phosphor. If the phosphor is too thin, then more than a desired amount of the blue light emitted by the LED will penetrate through the phosphor, and the combined LED—phosphor system light output will appear bluish, because it is dominated by the output of the blue LED. In contrast, if the phosphor is too thick, then less than a desired amount of the blue LED light will penetrate through the thick YAG:Ce phosphor layer. The combined LED-phosphor system will then appear yellowish, because it is dominated by the yellow output of the YAG:Ce phosphor.
Therefore, the thickness of the phosphor is a critical variable affecting the color output of the prior art system. Unfortunately, it is difficult to control the precise thickness of the phosphor during large scale production of the blue LED—YAG:Ce phosphor system. Variations in phosphor thickness often result in the system output being unsuitable for white light illumination applications, causing the color output of the system to appear non-white (i.e., bluish or yellowish), which leads to an unacceptably low blue LED—YAG:Ce phosphor system manufacturing yield.
The blue LED—YAG:Ce phosphor system also suffers from the halo effect due to the separation of blue and yellow light. The LED emits blue light in a directional fashion. However, the phosphor emits yellow light isotropically (i.e., in all directions). Therefore, when the light output by the system is viewed straight on (i.e., directly at the LED emission), the light appears bluish-white. In contrast, when the light output is viewed at an angle, the light appears yellowish due to the predominance of the yellow phosphor emission. When the light output by such a system is directed onto a flat surface, it appears as a yellowish halo surrounding a bluish area. The present invention is directed to overcoming or at least reducing the problems set forth above.